Silicon carbide, (SiC), is an important ceramic material for technological applications at extreme temperatures due to its exceptional physical and mechanical properties, such as high hardness, high thermal conductivity, low thermal expansion and resistance to erosion, corrosion and oxidation. SiC is also used as a reinforcement material in metal matrix composites such as aluminum.
Components fabricated from SiC materials have surfaces that come close to the hardness of diamond and possess excellent resistance to abrasion.
Recently, SiC nanowires and nanorods have attracted interest because of their novel physical properties resulting from quantum confinement. The electrical and optical properties due to low-dimensional nanostructures can be tailored for potential applications in nanoelectronics, nanosensors, and biotechnology. Nanocrystalline materials have mechanical properties that are largely governed by their ultimate sizes due to their large surface areas where most of the atoms are localized.
Consequently, it is possible to produce nanocrytalline/nanorod composites that are superhard materials that have promise for applications in the emerging field of miniaturized moving parts in microelectro-mechanical systems. In solid state electronic devices, quantum well (QW) structures play an important role where the charge carriers are confined at a nanometer length scale.
Recently, to avoid the effects of different chemical species in hetrostructure superlattice devices, SiC has been proposed as a promising candidate material of choice due to the presence of two stable and well-understood polytype phases. These α (4H) and β (3C) phases provide a variation of 1 eV energy gap. It was proposed that the 3C inclusions in 4H or 6H SiC behave like quantum wells. In addition, it is very promising material for power electronics and biomedical applications due to its high breakdown voltage and chemical inertness, respectively.
Discovery of new forms of SiC such as nanoporous structures have opened new horizons of applications in electronics. In addition, nanocrystalline SiC can have important applications in gem, optical, and metallurgical polishing, and Ni—SiC composite coatings for integrated circuit engine components.
Silicon carbide has many polytypes arising from the different scheme of stacking layers of C and Si atoms; the most common (α-SiC, 4H) is formed at temperatures greater than 1700° C. and has a modified hexagonal crystal structure (Wurtzite). The beta configuration (β-SiC, 3C), exhibits a zinc-blende crystal structure (diamond), and can be formed at temperatures below 1700° C. Due to the close proximity of silicon and carbon on the periodic table, the silicon to carbon bonds are highly covalent in nature.
In many of the applications for SiC nanostructures, large quantities are required and must be produced using a simple, inexpensive method. It is also important to note that currently there is a significant problem in sustainability due to the large quantities of rice husk that are a byproduct of white rice. The elemental composition of rice consists of elements such as Si, C, Fe, Mn, Ca etc. Because large quantities of rice are being consumed every year generating millions of tons of rice husks per year, disposing this agricultural waste is a big challenge.
Burning the rice husks in air only produces the extremely fine silica ash which poses health hazard. Therefore it is important to identify a means to successfully eliminate this waste, or better yet, repurpose it towards a useful end.
It has been shown that rice husk material provides an appropriate precursor material for the formation of SiC nanostructures via various techniques as well as from other methods. Silicon carbide can be produced by processes involving multiple steps consisting of heating rice husks in an inert atmosphere to temperatures higher than 1300° C. A single step method also was adopted by using plasma reactor using graphite electrodes.
In this disclosure, we describe a novel, simple, and single-step process in which raw rice husks, sorghum, peanuts, walnuts, almonds, pistachios, nut shells, maple leaves, fruit pits such as from dates, peaches, mango, and corn husk materials and others that contain silica can be converted directly to a collection of cubic β-SIC nanostructures using a method involving rapid heating in a vacuum using conventional heating or a millimeter-microwave beam that increases the localized temperature up to 1900° C.